1. Field of the Invention
The invention relates to the use of a phase shifting mask to improve resolution in the fabrication of sub-micron integrated circuits and more particularly to the fabrication and use of a quantum phase shift mask without chrome and a quantum phase shift build-on blank without chrome. The quantum phase shift build-on blank is readily inspected and can be easily stored for later completion and use.
2. Description of Related Art
As optical lithography advances to 0.5 to 0.35 microns and below new technologies are needed to improve the resolution of the imaging lens. Phase-shifting photomasks have been used to improve resolution as well as depth of focus. The use of phase-shifting photomasks are described in a number of places, for example U.S. Pat. No. 5,045,417 to Okamoto or U.S. Pat. No. 5,268,244 to Yoo. A transparent coating of thickness t=0.5L/(n-1) (L=wavelength, n=index of refraction) is placed on a mask. The phase of the electric field of the light passing through the coated area is delayed 180.degree. compared to the electric field of light passing through uncoated regions. At the boundary between the coated and uncoated regions the electric fields cancel producing an electric field null as well as an intensity null. This intensity null produces a sharper image as well as improved depth of focus at the integrated circuit wafer.
Phase shifting techniques have been described using several configurations. U.S. Pat. No. 5,190,836 to Nakagawa et al describes a reflection type photomask using phase shifting techniques. U.S. Pat. No. 5,194,345 to Rolfson describes rim phase shifters wherein phase shifting material is deposited on top of opaque material and extends beyond the opaque material in an overhang configuration thereby improving image resolution. U.S. Pat. No. 5,194,344 to Cathey, Jr. et al, U.S. Pat. No. 5,194,346 to Rolfson et al, U.S. Pat. No. 5,208,125 to Lowrey et al, U.S. Pat. No. 5,217,830 to Lowrey, and U.S. Pat. No. 5,225,035 to Rolfson describe various types of phase shifting masks and techniques. "LITHOGRAPHY'S LEADING EDGE, PART 1: PHASE-SHIFT TECHNOLOGY" published in Semiconductor International February 1992 pages 42-47 shows a number of phase shifting masks and techniques including a chromeless shifter.
While improvements have been made in image resolution and depth of field, the linear dimensions used in integrated circuit technology call for even greater improvements in resolution and increased depth of field.